Investigating the role of undercoordinated Pt sites at the surface of layered PtTe2 for methanol decomposition

Transition metal dichalcogenides, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been pursued in the design of ideal catalysts. Nevertheless, their structurally perfect basal planes are typically inert; their surface defects, such as under-coordinated atoms at the surfaces or edges, can instead serve as catalytically active centers. Here we show a reaction probability > 90 % for adsorbed methanol (CH3OH) on under-coordinated Pt sites at surface Te vacancies, produced with Ar+ bombardment, on layered PtTe2 — approximately 60 % of the methanol decompose to surface intermediates CHxO (x = 2, 3) and 35 % to CHx (x = 1, 2), and an ultimate production of gaseous molecular hydrogen, methane, water and formaldehyde. The characteristic reactivity is attributed to both the triangular positioning and varied degrees of oxidation of the under-coordinated Pt at Te vacancies.

Transition metal dichalcogenides, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been pursued in the design of ideal catalysts.Nevertheless, their structurally perfect basal planes are typically inert; their surface defects, such as under-coordinated atoms at the surfaces or edges, can instead serve as catalytically active centers.Here we show a reaction probability > 90 % for adsorbed methanol (CH 3 OH) on under-coordinated Pt sites at surface Te vacancies, produced with Ar + bombardment, on layered PtTe 2approximately 60 % of the methanol decompose to surface intermediates CH x O (x = 2, 3) and 35 % to CH x (x = 1, 2), and an ultimate production of gaseous molecular hydrogen, methane, water and formaldehyde.The characteristic reactivity is attributed to both the triangular positioning and varied degrees of oxidation of the under-coordinated Pt at Te vacancies.Two-dimensional transition metal dichalcogenides (TMDs) have attracted considerable interest, owing to their distinctive electronic properties [1][2][3][4][5][6][7][8] and highly tunable surface reactivity [9][10][11][12][13][14][15][16] .TMDs, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been a goal in the design of ideal catalysts.Structurally perfect basal planes of TMDs are typically inert whereas their surface defects, such as unsaturated coordinative atoms at the surfaces or edges, become catalytically active centers.Many studies on MoS 2 , one of the most developed TMDs, have demonstrated that by controlling the density of sulfur vacancies (active centers) at MoS 2 surface, its reactivity toward varied reactions can be manipulated [13][14][15][16][17][18][19][20][21] .For instance, the sulfur vacancies at MoS 2 surface, in spite of varied generation approaches (plasma 10 , ion bombardment 13 or chemical etching 14 ), facilitated hydrogen evolution reaction, through the mechanism involved with altered surface electronic structures and boosted electric conductivity 10,13,14 .Ptbased catalysts, including single crystals, nanoclusters, alloys, and electrodes, have been extensively investigated in both heterogeneous catalysis and electrocatalysis, because of their superior catalytic properties  . Neverheless, the catalytic properties of Ptbased TMDs have been little explored 11,[43][44][45] , even though they could possess better chemical and structural stability than commercial Ptbased catalysts such as supported Pt nanoclusters.The present study aims to shed light on the catalytic properties of Pt-based TMDs and their potential as catalysts.
We studied the decomposition of methanol (methanol-d 4 ) on layered PtTe 2 with various surface-probe techniques under both ultrahigh-vacuum (UHV) and near-ambient-pressure (NAP) conditions.Methanol (CH 3 OH) decomposition serves as the principal reaction in direct methanol fuel cells (DMFCs), a promising device that converts methanol efficiently to electricity [46][47][48][49][50][51][52] , and the liberation of hydrogen from the decomposition leads to viable generation of hydrogen, another clean source of energy 53,54 .PtTe 2 , as a group-10 TMD material, draws much attention for its tunable bandgap, high charge mobility, and ultrahigh air stability 1,3,[5][6][7]11,43,44 , but its surface reactivity is little understood. We generted the surface defects on PtTe 2 and controlled their concentration with Ar ion (Ar + ) bombardment.The surface structures of PtTe 2 were characterized using scanning tunneling microscopy (STM) and synchrotron-based photoelectron spectroscopy (PES), while the reactions, intermediates, and products, were monitored using PES, near-ambient-pressure photoelectron spectroscopy (NAP-PES) and near-ambient-pressure mass spectroscopy (NAP-MS).We also performed calculations based on density functional theory (DFT) to explore the mechanisms in detail.
The results showed that under-coordinated Pt (denoted as Pt uc ) at the PtTe 2 surface served as active sites for the reaction.Both dehydrogenation and C-O bond scission occurred for adsorbed methanol on the Pt uc sites, leading to the formation of CH x O* (x = 2 and 3; * denotes adsorbates) and CH x * (x = 1 and 2) as major intermediates and finally production of gaseous molecular hydrogen, methane, water, and formaldehyde via various processes.Gaseous formaldehyde (CH 2 O (g) ) was rarely observed from methanol decomposition on either Pt single crystals or supported Pt nanoclusters; meanwhile, CO* (or CO (g) ), a common product from Pt-based catalysts [22][23][24][25][26]30,[33][34][35][36]42 , was not produced in the present reaction; the pathway of C-O bond scission in the present reaction accounted for a significant proportion (reflected on a considerable quantity of produced CH x *), which also contrasts with its minor role on supported Pt clusters 34 . The catalytc nature of Pt uc sites on PtTe 2 thus differs from that of typical Pt-based catalysts.Moreover, the reactivity depended notably on the Pt uc concentration.At a small Pt uc concentration (≤ 10 %), methanol on the Pt uc at surface Te vacancies, the dominating surface defects, decomposed at a great probability (> 90 %).With increased Pt uc concentration (10 − 20 %), the probability decreased as the probability of decomposition to CH x O* was selectively decreased, attributed to structurally different Pt uc generated by extended Ar + bombardment.Nevertheless, in either case, the reaction probability (≥ 80 %) exceeds those on Pt single crystals 22,25 and supported Pt clusters 34 .We propose, with the support of DFT modeling, that both structural and electronic effects play essential roles in determining the observed catalytic properties.These results suggest that a PtTe 2 surface with the Pt uc at surface Te vacancies can serve as a superior catalyst for methanol decomposition; its catalytic selectivity can be controlled with the surface structures manipulated by varied Ar + bombardment.

Structural characterization with STM and PES
We utilized STM to characterize the surface structures of layered PtTe 2 before and after the treatment of Ar + bombardment.The as-cleaved surface was generally very flat and had few defects, as illustrated in Fig. 1a; the hexagonally arranged white spots were topmost Te atoms imaged, and several types of intrinsic defects observed previously with STM 55 , such as Te and Pt vacancies, were also observed in the present sample (Fig. S1).After controlled Ar + bombardment, the surface Te vacancies were evidently increased, together with few small clusters on the surface; the size of the vacancies varied from single-to multiple-Te vacancies (Fig. 1b) and the proportion of larger vacancies increased with the bombardment time (Fig. S1).Continuing to increase either the bombardment time or the Ar + kinetic energy generated more not only the surface Te vacancies but also structural variations, such as island edges and re-deposited atoms (Fig. 1c, d).With the aim of correlating structures with reactivity, we chose a kinetic energy of 0.5 keV for incident Ar + and a reduced Ar + dosage (the sample current multiplied by bombardment time) to control structural complexity for reaction experimentsthe Te vacancies were produced as the main surface defects while the surface crystallinity (monitored with the RHEED measurements, Fig. S2) was largely sustained.
Fig. 1e-h exemplify a high-resolution image for two single-Te vacancies and the corresponding line profile across one of the vacancies.The vacancy depth about 0.1 nm (Fig. 1f), similar to that of an intrinsic Te vacancy at the surface (Fig. S1), and the well overlap of the top Te atoms and vacancies in the model with the imaged ones (Fig. 1g) suggest that the vacancies were formed by the removal of the topmost Te atoms, i.e., the surface Te vacancies.As the positions of the surface Te atoms neighboring the vacancies remained nearly unchanged (Fig. 1g) and as they were mainly bonded to the underlying Pt, the generation of these Te vacancies altered little the positions of underlying Pt atoms.The well match of the STM image with the DFTsimulated one (Fig. 1h), produced based on the vacancy model in Fig. 1g, corroborates the vacancy structure.Small Ar + dosages removed mainly the surface Te while left the underlying Pt atoms unaltered.Our DFT modeling for the reactions discussed below also adds weight to the argument.The small clusters in Fig. 1b likely correspond to redeposited atoms (0.15 nm, one-atom high) after Ar + bombardment.As they were much fewer than the surface Te vacancies at small Ar + dosages, for which the reaction experiments were primarily performed, their contribution to the observed reactions was considered limited even though they could initiate or assist (as adsorption sites) reactions.
Our PES spectra further characterized these surface defects observed with STM.Fig. 2a exemplifies the comparison of the PES spectra of Pt 4 f core level for the layered PtTe 2 as cleaved and bombarded by Ar + .The spectrum from as-cleaved PtTe 2 (upper panel of Fig. 2a) shows a doublet, Pt 4f 5/2 and Pt 4f 7/2 , centered at the binding energies (BE) 75.9 and 72.6 eV respectively, which corresponds to intact Pt-Te chemical bonds in PtTe 2 .Meanwhile, the PES spectrum from Ar + -bombarded PtTe 2 shows an additional feature at a BE slightly smaller than that of the main Pt feature, i.e., at 71.8 eV for Pt 4f 7/2 and 75.1 eV for Pt 4 f 5/2 (indicated by red fitted curves in the bottom panel).The smaller BE for Pt 4 f implies less oxidized Pt, corresponding to under-coordinated Pt (Pt uc ), at the PtTe 2 surface and therefore the removal of Te atoms by Ar + bombardment.In contrast, the Te 4d signals were insensitive to the removal of neighboring Te atomsthe line shape of Te 4d doublet altered little after Ar + bombardment (Fig. 2b), since the surface Te is mainly bonded to the underlying Pt.Both the Pt 4f and Te 4d lines from Ar + -bombarded PtTe 2 shifted negatively (approximately 0.1 eV as shown in Fig. 2a, b), likely because the bombardment induced a band-bending effect.As the spectral features for varied Ar + dosages are similar despite varied intensities of the Pt uc features (Fig. S3), the Pt uc signals correspond to the Pt uc at surface Te vacancies (denoted as Pt uc -Vac) and also to those at other surface defects (denoted as Pt uc -Ex) generated by greater Ar + dosages.Our experiments show that the number or concentration of Pt uc can be controlled with Ar + dosage and Ar + kinetic energy.Fig. 2c plots the ratio of the Pt uc and total Pt signals (denoted as Pt uc /Pt), measured by the integrated intensities of the red and black fitted curves (Fig. 2a), as a function of the Ar + dosage.The Pt uc /Pt ratio increased almost in a linear fashion with the Ar + dosage, despite varied Ar + kinetic energies.As about 90 % of the Pt signals came from the top two PtTe 2 bilayers (according to the escape depth of the Pt 4 f photoelectrons) and as Ar + bombardment at a small dosage removed primarily the surface Te (Fig. 1), each Pt uc /Pt ratio corresponds to a derivable concentration of surface Pt uc .Notably, the rate of increase of the Pt uc /Pt ratio depended on the Ar + kinetic energy, since the cross-section of removing surface Te varied with the Ar + kinetic energy.The Ar + at 0.5 keV was chosen to prepare the sample, as it exhibited the best controllability in producing small Pt uc concentrations (small Pt uc /Pt ratios), warranting the surface Te vacancies as the dominating surface defects for catalytic studies (Fig. S1).Our reaction experiments were primarily performed on the PtTe 2 with Pt uc /Pt ratios ≤ 0.13 (Ar + dosage ≤ 4.5, Fig. 2c), corresponding to the Pt uc concentration ≤ 20 %; the Ar + dosages (0.5 keV) at 1 and 6 in Fig. 2c produced the surfaces resembling those shown in Fig. 1b, c respectively.

Methanol decomposition monitored with PES, NAP-PES and NAP-MS
The methanol reactions were characterized primarily by PES spectra; with the spectra, we monitored the evolution of surface species with temperature and Pt uc concentrations.Fig. 3a, b compare the C 1 s spectra for methanol adsorbed on as-cleaved and Ar + -bombarded PtTe 2 (Pt uc /Pt ratio = 0.07) at 145 K and annealed stepwise to selected temperatures.The dominant feature at 145 K on either surface, centered about 286.5 eV, is assigned to the C 1 s line of monolayer methanol adsorbed on PtTe 2 (top in Fig. 3a, b), since multilayer methanol desorbed near 130 K 34,56,57 .On the as-cleaved PtTe 2 surface, the methanol signals decreased with increased temperature and vanished at 200 K, reflecting the desorption of methanol (Fig. 3a).In contrast, on the PtTe 2 with a number of surface Pt uc produced by Ar + bombardment, new features grew at 283.8 -285.1 eV above 160 K, at the expense of attenuating methanol feature at 286.5 eV (Fig. 3b).The results suggest that with increased temperature, a fraction of methanol desorbed whereas the other fraction decomposed and produced new carbon species.As the as-cleaved PtTe 2 has very limited surface defects, the contrasting results indicate that the structurally perfect basal plane of layered PtTe 2 is catalytically inert but the surface Pt uc , mostly at the Te vacancies (Fig. 1b, e), i.e.Pt uc -Vac, are reactive toward methanol decomposition.The products at elevated temperature consisted of three carbon species: CH x O* (x = 1-3), CH x * (x = 1-3) and atomic carbon (C*), corresponding to the C 1 s lines centered at 285.1 [58][59][60][61][62][63][64] , 283.8 [65][66][67] and 284.2 eV (Fig. S4a), respectively.CO* was not expected because our adsorption experiments showed no CO adsorbed on such a defective PtTe 2 surface even at 145 K. (purple) began to emerge above 200 K and became the major species at and above 300 K.The formation of CH x O* and CH x * indicates that dehydrogenation and C-O bond scission, the two competing processes of methanol decomposition 30,34,35,68 , were both catalytically activated at such low temperature on the surface Pt uc sites.A fraction of CH x O* and CH x * must have desorbed at elevated temperature, since the quantity of the remaining C* was not comparable to that of the produced CH x O* and CH x * (Fig. 3e).Our NAP experiments presented below indicate that the desorbing carbon species consisted largely of CH 2 O (g) and CH 4(g).
To reveal how the catalytic properties of PtTe 2 surface vary with the concentration of surface Pt uc , we plot the probabilities of conversion of adsorbed monolayer methanol to CH x O* (red circles) and CH x * (blue) as a function of the concentration of surface Pt uc , shown in Fig. 4a.The conversion probability was derived from the ratio of the C 1 s intensities of CH x O* (or CH x *) to monolayer methanol (145 K); the former was measured at 180 K because the maximum quantity of CH x O* (or CH x *) was produced around 180 K; only monolayer methanol was considered because it was directly in contact with the PtTe 2 surface.The concentration of surface Pt uc was estimated according to the measured Pt uc /Pt ratios (Fig. 2), as mentioned above; the percentage corresponds to the fraction of Pt uc in the total amount of Pt in the top PtTe 2 bilayer.For CH x * (blue circles), the conversion probability of monolayer methanol increased almost in linear proportion to the Pt uc concentration, corroborating that Pt uc served as reactive sites for the C-O bond scission.Nevertheless, the trend becomes complicated for CH x O* (red).The conversion probability increased linearly at a Pt uc concentration ≤ 10 %, while became saturated (10 -20 %) or even decreased at a greater Pt uc concentration (> 20 %).At greater Pt uc concentrations (> 10 %) the Pt uc structurally different from Pt uc -Vac had grown and accounted for a fraction of total Pt uc , although they were not resolved in the Pt 4 f spectra.Such Pt uc corresponded to Pt uc -Ex; they possessed different catalytic properties so initiated a separate reaction pathway, for which the C-O bond scission was more facilitated so CH x O* became instable and decreased.The conversion probability of methanol adsorbed on the Pt uc sites (denoted as methanol/Pt uc ) as a function of the Pt uc concentration shows clearly the evolving catalytic selectivity (Fig. 4b).The fraction of methanol/Pt uc in total monolayer methanol was estimated to be that of Pt uc in total amount of Pt in the top PtTe 2 bilayer (the Pt uc concentration), by assuming that monolayer methanol adsorbed uniformly on the PtTe 2 surface.As shown in Fig. 4b, the conversion probability of methanol/Pt uc to CH x * varied little with the Pt uc concentration, remaining near 35 %; in contrast, that to CH x O* was approximately 60 % at a Pt uc concentration ≤ 10 % but decreased continuously at a Pt uc concentration > 10 %.The observation does not simply suggest that the formation of CH x O* (dehydrogenation) was structure-sensitive whereas that of CH x * (C-O bond scission) was not [69][70][71] because the productions of these two intermediates were not necessarily separate -CH x * could be produced largely via the C-O bond scission of CH x O*.Besides, both the dehydrogenation and C-O bond scission were considered sensitive to structures [69][70][71] .The present result is likely due to enhanced C-O bond scission, which decreases CH x O* but increases CH x *, and obstructed dehydrogenation, which decreases both CH x O* and CH x *, on the Pt uc -Ex sites mentioned above.Our STM measurements show that at a greater Ar + dosage (Pt uc concentration > 10 %), the generated surface defects include not only the Te vacancies (Fig. 1b) but also other defects, such as edges of PtTe 2 patches (islands) and Pt-Te nanoclusters formed by nucleation of redeposited Pt and Te (exemplified in Fig. 1c and Fig. S4b).We thus associated the Pt uc -Ex with the Pt uc at these two surface defects.Nevertheless, we note that the total reaction probability was remarkably greatabout 95 % at a Pt uc concentration ≤ 10 % and remaining greater than 80 % even at a concentration near 20 %.In either case, the reaction probability exceeded that for methanol on either Pt singlecrystal surfaces (10 %) 22,25 or supported Pt clusters (60 -70 %) 34 .Additionally, the probability of conversion to CH x * (35 %) was also evidently greater than that on Pt clusters (<10 %) 34 ; the C-O bond scission pathway apparently played an important role in the present reaction.
The above experiments demonstrate that the catalytic reactivity of layered PtTe 2 is controllable by the surface Pt uc under UHV conditions.To unveil their catalytic performance under "real-world" conditions 72 , we investigated methanol reactions on layered PtTe 2 near ambient pressure.Fig. 5a exemplifies the NAP-PES spectra of C 1 s core level from PtTe 2 bombarded by Ar + (Pt uc /Pt ratio = 0.10) and subsequently exposed stepwise to selected pressures of methanol at 300 K; 300 K was used because the desorption and further decomposition of products (intermediates) already occurred (Fig. 3).The as-bombarded PtTe 2 surface was free of carbon contamination, indicated by the absence of C 1 s signals (the bottom of Fig. 5a); increasing methanol pressure to 10 À4 mbar, a small C 1 s line arose around 284.2 eV and continued to grow with increased pressure; at 10 À3 mbar and above, a shoulder centered about 283.0 eV (Fig. 5a,b) also grew.The former is assigned to C*, while the latter to CH x *, both of which resulted from decomposed methanol on PtTe 2 .The reactions must have occurred on the surface Pt uc sites (largely Pt uc -Vac sites at this small Pt uc concentration) as the experiments on as-cleaved PtTe 2 (with scarce surface Pt uc ), as a comparison, showed negligible C 1 s signals.This observation agrees with that on the Ar + -bombarded PtTe 2 surface under UHV conditions, in which C* and CH x * were primary remaining species on the surface at 300 K (Fig. 3e).These C 1 s signals increased with methanol pressure, since more methanol decomposed on the PtTe 2 surface at greater methanol pressures.They decreased at 0.1 mbar (second from the top in Fig. 5a) as the photoelectrons were attenuated by the increased pressure; at such a great pressure, the C 1 s feature resulting from gaseous methanol also appeared at 288.5 eV 35,73 .Notably, the ratio of CH x * to C* signals increased with methanol pressure but altered little with the decreasing pressure from 0.1 to 10 À7 mbar.The ratio was affected little by the pressure-induced signal attenuation but determined by the composition of carbon species on the surface.The fraction of CH x * in total surface carbon species became greater at a greater pressure or C* concentration.We speculate that a greater C* concentration at the Pt uc sites suppressed the dehydrogenation of CH x * by altering the electronic properties of Pt uc and/or adsorption configurations of CH x *.
The corresponding gaseous products from the methanol reactions near ambient pressure were monitored with NAP-MS.The gaseous products also confirm that the reactivity and selectivity of Pt uc sites depend on the Pt uc concentration.Fig. 5d shows the gaseous products (at methanol pressure 10 -2 mbar) as a function of the Pt uc concentration.For a Pt uc concentration <20 %, the products increased generally with the Pt uc concentration, which is consistent with the trend shown in Fig. 4a.The production of CD 2 O (g) dropped dramatically when the Pt uc concentration increased above 20 %, in agreement with the decrease of CH x O* (Fig. 4a).The produced D 2(g) also decreased, indicating the selectively suppressed dehydrogenation to CH x O*.In contrast, CD 4(g) /D 2 O (g) decreased only slightly at a Pt uc concentration > 20 %, as CD x * increased at 180 K (Fig. 4a); the proportion of CD x * desorbing as CD 4(g) could vary with the Pt uc concentration.The dependence is associated, as discussed above, with the composition of Pt uc -Vac and Pt uc -Ex evolving with the Pt uc concentration.

DFT modeling
Our first-principles DFT modeling aimed to elucidate the key mechanisms behind the observed reactions.We established a Tedivacancy model by removing two adjacent Te atoms at the topmost layer in order to mimic a PtTe 2 surface at which the reactive sites consist primarily of the Te vacancies (Fig. 1b,e).The model is applicable to the cases of Pt uc concentration ≤ 20 % because the Te vacancies remained as the main surface defects despite the growth of the Pt uc -Ex at a Pt uc concentration > 10 %.Although the Te vacancies of various sizes were observed, the divacancy model suffices to represent the key features of the reactions on the Pt uc -Vac sites.A divacancy site has five Pt uc -Vac: one loses two Te-Pt bonds in the bilayer structure (denoted as Pt uc2 -Vac), corresponding to the coordination to four Te, and the other four lose one Te-Pt bond (Pt uc1 -Vac), illustrated in Fig. 6a.The structural modeling confirms that the underlying Pt atoms remain nearly at the same positions after the removal of surface Te.We considered three main reaction processes in the methanol decomposition: dehydrogenation from either oxygen (red arrows) or carbon (green arrow), and C-O bond scission (deoxygenation and dihydroxylation; black arrows), presented in Fig. 6b.The adsorption configurations and energies of methanol and its decomposition fragments are presented in Fig. S5.The modeling shows that a methanol molecule adsorbs on a Pt uc -Vac site with an O-Pt binding configuration and adsorption energy −1.02 eV, which is evidently stronger than that (−0.41 eV) on pristine PtTe 2 basal plane (Fig. S5).The Pt uc -Vac, therefore, has the potential to serve as a reactive center on PtTe 2 .The three main decomposition processes at varied stages at the divacancy sites were calculated and compared.For the first step of decomposition, the barrier for dehydrogenation from oxygen (0.66 eV) of CH 3 OH* was significantly smaller than those for dehydrogenation from carbon (1.42 eV) and dehydroxylation (1.82 eV).The comparison suggests the preferential formation of methoxy (CH 3 O*), agreeing well with the observed selectivity that methanol decomposed via a pathway to produce more CH x O* than CH x * at a small Pt uc concentration (Fig. 4a).The great difference between the energy barriers for desorption and dehydrogenation to CH 3 O* (1.02 vs. 0.66 eV) also explains the great conversion probability for methanol adsorbed on the Pt uc -Vac site (Fig. 4b).CH , as observed in NAP-PES spectra (Fig. 5a) and NAP-MS spectra (Fig. 5c, d) respectively.Details of the calculated energy barriers are provided in our Supplementary Information (Figs.S5 -S18).
After the C-O bond scission of CH 2 OH*, the produced OH* either diffuses away from the active sites or combines with H* to desorb as H 2 O (g) (Fig. 5c, d), so its "poisoning effect" is insignificant.The calculated activation energy for OH* to migrate to an intermediate adsorption site on Te at the edge of divacancy amounts to 0.95 eV; their diffusion barrier on the basal plane is even smaller (0.36 eV), as shown in Fig. S19.These diffusion barriers are smaller than that for the C-O bond scission of CH 2 OH*, so with the progress of methanol decomposition, the produced OH* can diffuse readily to other sites to prevent the active sites from obstruction.
The above modeling shows that Pt uc -Vac in the divacancy model activates the decomposition in a coordinative manner; the adsorbates (CH 3 OH*, its decomposition intermediates and fragments) are bonded mostly to two or three Pt uc -Vac in varied decomposition processes (Figs.S5 -S18).Fig. 7a shows the simulated dehydrogenation of CH 3 OH* to CH 3 O* as an example.CH 3 OH* first adsorbs on the Pt uc2 -Vac through its O and then undergoes the scission of O-H bond.At the final stage, H* is bonded to one of Pt uc1 -Vac and CH 3 O* to both Pt uc1 -Vac and Pt uc2 -Vac.It is noted that neither CH 3 O* nor H* is bonded to Te near the vacancy in the process, since they adsorb weakly on these Te sites, like CH 3 OH* and its other decomposition intermediates or fragments (Fig. S5).As Te atoms at PtTe 2 surface are mainly bonded to the underlying Pt 6,11,55 , instead of their neighboring Te atoms, the removal of surface Te atoms altered little the electronic properties of Te atoms surrounding the vacanciesthe bonding of these surface Te atoms remains saturated.As a result, they were not heavily involved in the methanol decomposition.In the divacancy model, Pt uc2 -Vac appears to be more active than Pt uc1 -Vac, indicating that the processes are mostly centered around Pt uc2 -Vac in spite of varied reaction pathways (Figs.S5  -S18).The Pt uc -Vac are like separated Pt single atoms, triangularly positioned and oxidized to different extents dependent on their bonding to Te.The number of coordination (to Te) or missing Te-Pt bonds of Pt uc -Vac determines its electronic properties and hence catalytic properties.To illuminate the effect of the coordination number of Pt uc -Vac on the activity, a tri-vacancy model with a Pt uc3 -Vac in the middle of the tri-vacancy site, which was also likely formed on the Ar +bombarded PtTe 2 surface, was introduced to simulate the reactions.The results show that the adsorption energies of CH 3 OH*, CH 3 O* and CH 2 O* on the tri-vacancy site were slightly increased by 0.05 ~0.10 eV, and the energy barrier for dehydrogenation of CH 3 OH* to CH 3 O* remained similar, whereas that of CH 3 O* to CH 2 O* was reduced by 0.12 eV (Figs.S20-S22).Consequently, the formation of CH 2 O* on this tri-vacancy became more probable.Reducing the coordination to Te enhances the activity of Pt uc -Vac.
The corresponding electronic structures reflect the same trend.The measured and calculated local densities of states (LDOS) near the Fermi level of Pt uc0-3 -Vac (Fig. 7b) show consistently that with decreased coordination number, the Pt uc0-3 -Vac at PtTe 2 surface become more metallic, reflected on the enhanced LDOS near the Fermi level; meanwhile, their d-band centers shift toward higher energies (Fig. 7c).Both results are indicative of enhanced catalytic reactivity 74,75 .The LDOS of Pt uc1-3 -Vac at PtTe 2 surface and Pt at Pt(111) surface are evidently different (Fig. S23), accounting for their different catalytic behaviors.Previous studies indicate that the spatial distribution and orientation of frontier orbitals of single-atom catalysts are well correlated with adsorption and catalytic activities 76 .We note that the spatial distributions of frontier orbitals (in the energy ranging from -0.25 eV to the Fermi level) of Pt uc -Vac at PtTe 2 surface expanded with decreased coordination number, as plotted in the insets of Fig. 7c.The expansion reflects a promoted probability of wave-function overlap, which is required for adsorption and catalytic activities 76 , so agrees with the enhanced reactivity indicated above.Thus, the characteristic reactivity https://doi.org/10.1038/s41467-024-44840-zNature Communications | (2024) 15:653 of the Pt uc -Vac arises from not only the peculiar structural (geometric) effecttriangularly positioned Pt uc -Vac, but also the electronic effect differently oxidized Pt uc -Vac.
The above modeling also indicates that enlarging the Te vacancies (increasing the number of more active Pt uc3 -Vac) promotes the main reaction processes shown in Fig. 6b, instead of altering the reaction pathway.It accordingly supports the argument that a separated reaction pathway reflected on either the decreased conversion probability to CH x O on Pt uc sites (Fig. 4b) or the decreased production of CD 2 O (g) and D 2(g) (Fig. 5d) at a greater Pt uc concentration was initiated by the Pt uc -Ex.The Pt uc -Ex consists largely of Pt uc at the edges of PtTe 2 patches (Fig. 1c and S4b) or in redeposited Pt-Te nanoclusters (Fig. 1c).The Pt uc -Ex at the edges of PtTe 2 patches (islands) are primarily the Pt uc with one missing Te-Pt bond (Pt uc1 -Ex), as illustrated in Fig. S24.Our DFT modeling for methanol on the edges of PtTe 2 patches (Figs.S24-S27) show that CH 3 OH* on the edge Pt uc1 -Ex sites prefers desorption to decomposition, in the light of evidently greater activation energies for the dehydrogenation to CH 3 O* (1.64 eV) and the C-O bond scission (2.07 eV) than that for desorption (0.57 eV).Notably, the activation energies are greater and the adsorption is weaker than those (Fig. 6b and Fig. S20) on the Pt uc2 -Vac (or Pt uc3 -Vac) in the divacancies (trivacancies).The result implies that increasing the edge Pt uc1 -Ex sites decreases the average conversion probability (to both CH x O* and CH x *) on Pt uc sites, which explains the observed behavior of CH x O* (Fig. 4b).As a result, the Pt uc -Ex on the redeposited Pt-Te nanoclusters should be responsible for the production of CH x *, to match the nearly constant probability of conversion to CH x * (Fig. 4b).Earlier studies showed that the C-O bond scission of methanol and subsequent production of CH 4 were promoted on supported nanoscale Pt clusters 34 ; meanwhile, the dehydrogenation to CO also occurred, which contrasts with the present observation.Therefore, the Pt-Te nanoclusters, instead of pure Pt clusters, are more likely the structures to yield CH x *.These Pt-Te nanoclusters had no long-range structural ordering since our RHEED measurements showed no additional diffraction patterns.As they were formed by nucleation of redeposited Pt and Te, the Pt atoms in the clusters were not bonded exclusively to Te so not separated as far as those in PtTe 2 .The Pt uc -Ex at the cluster's surface thus differs structurally from the Pt uc -Vac and the Pt uc -Ex at edges of PtTe 2 patches discussed above.Small Pt aggregates likely formed in the Pt-Te clusters so their surface Pt uc -Ex exhibited reactivity partially resembling that of supported Pt nanoclusters 34 .

Discussion
With STM, PES, NAP-PES, NAP-MS, and DFT calculations, we studied methanol reactions on layered PtTe 2 under both UHV and NAP (up to 0.1 mbar) conditions.A structurally perfect PtTe 2 surface was inactive surface can serve as a superior catalyst toward methanol decomposition, with advantages of great catalytic reactivity and tunable selectivity.

Sample preparation
Single-crystal PtTe 2 was synthesized by the self-flux method.Highpurity Pt (99.99%, 0.35 g, 1.79 mmol) foils and a Te ingot (99.9999%, 4.35 g, 34.09 mmol), obtained from Ultimate Materials Technology Co., Ltd, were mixed in a ratio of 1:17 and sealed in a quartz tube under vacuum at ~5 × 10 -5 Torr.The tube was heated to 1000 °C in 12 hours, held there for 24 hours, and then slowly cooled to 500 °C at a rate of −3 to −5 °C.The excess Te was subsequently separated by centrifugation.
To improve the crystal quality and remove any residual Te, the crystal was sealed in an evacuated quartz tube again and heated at 450 °C for 100 hours.The average yield of PtTe 2 with this method was 90 -95% (based on Pt).The grown bulk crystal has a diameter near 8.0 mm; it was cleaved in situ before each experiment.

Characterization
The UHV experiments were conducted in UHV chambers with a base pressure in the regime of 10 −10 Torr.Ar + bombardment was performed with an acceleration energy of 0.5 −2.0 keV and under a pressure of 5 × 10 -6 Torr; 0.5 keV was chosen for the sample preparation in catalytic studies.The sample was maintained at 300 K for exposure to CH 3 OH and methanol-d 4 (CD 3 OD) at selected pressures.The highly pure CH 3 OH and CD 3 OD (Merck, 99.8%) were additionally purified by repeated freeze-pump-thaw cycles before being introduced into the experimental chambers.The gas exposure in the present work is reported in Langmuir units (1.0 L = 1 × 10 −6 Torr s).STM images were acquired at sample temperature 77 K (samples cooled from 300 K in an hour) in constant-current mode using an electrochemically etched tungsten tip, with a sample bias voltage (V s ) of −150 mV − −500 mV and a tunneling current (I t ) of 1.0 − 3.0 nA.The PES and NAP-PES experiments were conducted at the TLS BL09A2 and BL24A beamlines, respectively, at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan 77,78 .For the former, the incident photon beam (with a fixed energy 410 eV) was normal to the surface and photoelectrons were collected (Scienta R3000) at an angle of 58°from the surface normal; for the latter, the beam (with a fixed energy 380 eV) was incident 56°from the surface normal and the analyzer (SPECS NAP 150), equipped with a 4-stages differential pumping system, was placed normal to the surface.The energy resolution was estimated to be near 0.1 eV and the BE was referred to the Au 4 f core level at 84.0 eV of an Au substrate placed beside the investigated PtTe 2 sample.All spectra presented here were normalized to the photon flux.The gaseous reaction products were measured with NAP-MS, comprised of a quadrupole mass spectrometer (Hiden HAL201RC) and a doublydifferentially pumping system with a stainless steel tubing inlet terminated with an aperture (a diameter 0.75 mm) 3 mm from the sample.We used a full-range gauge mounted on the reaction chamber to calibrate the ion current at a selected m/z ratio to the partial pressure of the corresponding product.The presented partial pressures of the gaseous products were derived by subtracting the background, which was collected from as-cleaved PtTe 2 surface (inert toward methanol decomposition), under the corresponding methanol pressure.

DFT calculations
First-principles calculations were performed by using Vienna ab initio simulation package (VASP) 79,80 , in the framework of DFT with the projector-augmented wave (PAW) method 81 and the Perdew-Burke-Ernzerhof functional 82 type generalized gradient approximation exchange-correlation functional.For the 1T-PtTe 2 calculations, the energy cutoff was 330 eV and the energy convergence criterion was 10 -4 eV.A 12 × 12 × 1 Γ-centered k-point sampling was performed for the Brillouin zone integration until the relative energies converged to a few meV.The optimized lattice constant was 3.91 Å, in good agreement with a previous report of 3.9 Å 83 .We included weak van der Waals (vdW) interaction between adjacent two bilayers using the recently developed SCAN+rVV10 84 correction method that yielded excellent geometric and energetic results at a reasonable computational cost.The 1T-PtTe 2 layered structure was then modeled by a 6 × 6 supercell with three bilayers of 1T-PtTe 2 .The bottom bilayer was fixed and the remaining two bilayers were allowed to relax.The adsorption energy (E ads ) for each adsorbate was evaluated by the following standard formula: E ads = E A +S -E S -E A , where E A+S , E S , and E A denote the total energies of the adsorbed system, the clean PtTe 2 layered system, and the chemical potentials of the corresponding adsorbed species in a gas phase, respectively.The energy barriers were determined using the climbing image nudged elastic band (CINEB) method 85 .

Fig. 1 |
Fig. 1 | Charcterization of surface structures of layered PtTe 2 with STM.STM images for layered PtTe 2 a as-cleaved (V s = -250 mV, I t = 1.35 nA) and bombarded by Ar + with b 0.5 keV for 30 sec.(V s = −150 mV, I t = 1.20 nA) and c 180 sec., and with d 2.3 keV for 30 sec.(V s = −500 mV, I t = 2.90 nA); e the high-resolution image of two single-Te vacancies, f the line profile across a single-Te vacancy, g the overlap of the two single-Te vacancies model with the imaged ones and h the match of the STM image with the DFT-simulated one produced based on the vacancy model in (g).In g, light blue, blue and grey balls denote top, bottom Te and Pt atoms in the topmost PtTe 2 bilayer; the dash-line circles denote single-Te vacancies.In h, green balls denote the simulated images for the top Te atoms; the simulated image was derived with bias −0.1 V while the images obtained with −0.5 ~−0.1 V were similar.

Fig. 2 |
Fig. 2 | Charcterization of surface structures of layered PtTe 2 with PES.PES spectra of a Pt 4 f and b Te 4d core levels from layered PtTe 2 as cleaved and bombarded by Ar + (0.5 keV, 3 mins); c ratios of integrated intensities of Pt uc to Pt 4 f lines as a function of Ar + dosages.In a and b, gray circles denote the spectra and black lines the sum of fitted curves; the signals from intact Pt and under-coordinated Pt (Pt uc ) in the layered PtTe 2 are fitted with blue and red lines, respectively.In c, the Ar + dosages were calculated by sample current multiplied by sputtering time; the data obtained with Ar + kinetic energies of 0.5, 1.0, and 2.0 keV were represented by red, blue, and purple rhombuses respectively in the plot, and the error bars indicate the reproducibility.

Fig. 3 |
Fig. 3 | Reactions of methanol adsorbed on PtTe 2 monitored with PES.PES spectra of C 1 s core level for methanol (4.0 and 2.0 L) adsorbed on a as-cleaved and b Ar + -bombarded PtTe 2 at 145 K and annealed stepwise to selected temperatures; the illustration fits the spectra obtained at c 180 and d 260 K; e the integrated intensities of the fitted curves representing CH x O* (red circles), CH x * (blue) and C* (purple) signals as a function of temperature.In c and d, gray lines denote the spectra and black ones the sum of fitted curves; absorbed methanol, CH x O*, CH x *, and C* signals are fitted with green, red, blue, and purple lines, respectively.The dash lines in (e) are the guidance for eyes only.

Fig. 4 |
Fig. 4 | Varied reaction probabilities of methanol adsorbed on PtTe 2 surface with Pt uc sites.Probabilities of conversion to CH x O* (red circles) and CH x * (blue) of monolayer methanol adsorbed on a PtTe 2 surface, and those on b Pt uc sites as a function of the concentration of surface Pt uc .The dash lines are drawn to guide eyes only and the error bars indicate reproducibility.The insets (STM images) in (b) indicate the structural evolution with the concentration of surface Pt uc (Ar + dosages).

Fig. 5 |
Fig. 5 | Reactions of methanol on PtTe 2 under NAP conditions monitored with NAP-PES and NAP-MS.NAP-PES spectra of a C 1 s core level from Ar + -bombarded PtTe 2 (Pt uc /Pt ratio 0.10) exposed to varied pressures of methanol, as indicated, at 300 K; b the illustration of the fit to the spectrum obtained at methanol pressure 10 -2 mbar; the production of D 2(g) (m/z 4 u), D 2 O (g) /CD 4(g) (m/z 20 u) and CD 2 O (g) (m/z 32 u) from PtTe 2 at 300 K, as a function of c methanol-d 4 pressure (Pt uc /Pt ratio 0.10) and d Pt uc concentration (10 -2 mbar).In (b), the gray line denotes the spectrum, and the black one is the sum of fitted curves; CH x * and C* signals are fitted with blue and purple lines, respectively.In c and d, the error bars indicate the reproducibility.

Fig. 6 |
Fig. 6 | Schematic pathways of methanol reactions on a Te-divacancy site at PtTe 2 surface.a The atomic model for a Te divacancy at PtTe 2 surface.b Pathways of methanol decomposition on the Te-divacancy site at PtTe 2 surface.In b, the possible intermediates in their most stable configurations and their adsorption energies on the Te-divacancy site are presented (value at the upper-left corner of each panel).The sizes of Pt and Te atoms in the model are varied to illustrate their relative positions from top view.Brown and red hollow arrows indicate the dehydrogenation process from C and O atoms of methanol, respectively; black hollow and grey dashed arrows represent dehydroxylation and deoxygenation processes, respectively; the green hollow arrow indicates a hydrogenation process.For clarity, the arrows for the most plausible processes in the methanol decomposition are enlarged.The value beside each arrow is provided for the energy barrier of the specific process.

Fig. 7 |
Fig. 7 | Dehydrogenation processes of CH 3 OH* to CH 3 O* on a Te-divacancy site and electronic structures near the Fermi level of Pt uc -Vac.a Schematics illustrating the dehydrogenation processes (1 → 4) of CH 3 OH* to CH 3 O* on a Tedivacancy site; the corresponding energy profile is given in Fig. S7.b Comparison of the measured and calculated (inset) LDOS near the Fermi level of Pt uc0-3 -Vac.c Comparison of d-band centers of Pt uc -Vac at PtTe 2 surface and Pt at Pt (111) surface.In b, the measurements, dI t /dV s vs. V s , were conducted with scanning tunneling spectroscopy (STS), the arrows in the STM images (inset) indicate the locations where the STS measurements were performed, and the dash line in the calculated LDOS (inset) indicates the Fermi level.In c, the Pt under-coordination number, the x-axis, indicates the number of missing Te-Pt bonds of Pt uc -Vac.The insets show the corresponding spatial distributions of frontier orbitals (in the energy ranging from −0.25 eV to the Fermi level) of Pt uc -Vac (red spheres), the other atoms (yellow) at PtTe 2 surface, and Pt at Pt (111) surface (red).